This application is based upon and claims the benefit of priority of the prior Japanese Patent Applications No. 2000-176961 filed on Jun. 13, 2000, and No. 2001-40498 filed on Feb. 16, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a semiconductor pressure sensor having a diaphragm portion that is deformed due to applied pressure, strain gauges for generating electric signals in accordance with deformation of the diaphragm portion, and a circuit portion for detecting the electric signals from the strain gauges, which are formed on a semiconductor substrate composed of a first silicon substrate and a second silicon substrate with an insulation film interposed therebetween.
2. Description of the Related Art
In such a semiconductor pressure sensor, a diaphragm portion for detecting pressure and a circuit portion are integrated on a single semiconductor chip (semiconductor substrate). Conventionally, analog circuit elements such as bipolar transistors are mainly used as circuit elements that are integrated. On the other hand, there is a request of integrating analog circuits and digital circuits in accordance with the recent improvement of LSI processing and LSI devising technologies.
In such a situation, JP-A-4-103177 discloses a semiconductor pressure sensor in which a diaphragm portion, digital circuit elements and analog circuit elements can be integrated on a single semiconductor chip. In this semiconductor pressure sensor, the digital circuit elements such as CMOS (Complementary MOS) circuits are electrically isolated from each other through a LOCOS (Local Oxidation of Silicon) film on a silicon substrate, i.e., a SOI (Silicon On Insulator) wafer where the diaphragm portion is formed.
However, when the LOCOS film for isolating the digital circuit elements is formed on a thin-wall portion of the diaphragm portion, since the LOCOS film is thick, strain characteristics of the diaphragm portion, that is, sensor characteristics is adversely affected by the LOCOS film. Further, it is required to improve the withstand voltage property between the circuit elements to realize higher integration of the circuit elements.
On the other hand, recently, such a semiconductor pressure sensor has been used for various applications. For example, such a semiconductor pressure sensor has been used as an intake pressure sensor in a vehicle engine intake system or the like, which is liable to be disclosed to contaminants in a severe environment. Therefore, since it is feared that sensor characteristics are deteriorated due to contaminant adhesion to the sensor, environment resistance is required to be further improved.
In view of the foregoing problems, an object of the present invention is to restrict a LOCOS film from adversely affecting sensor characteristics in a semiconductor pressure sensor having a diaphragm portion for detecting a pressure. Another object of the invention is to improve sensor characteristics of a semiconductor pressure sensor including circuit elements isolated from each other by a LOCOS film.
According to the present invention, a semiconductor pressure sensor includes a first silicon substrate, a second silicon substrate, and an insulation film interposed between the first and second silicon substrates. The second silicon substrate has a recess portion, and a diaphragm portion is formed by the first silicon substrate as a bottom of the recess portion. A strain gauge is formed on the diaphragm portion, and a circuit portion for detecting an electric signal from the strain gauge is formed on the first silicon substrate at a portion other than the diaphragm portion. Further, a LOCOS film for electrically isolating the strain gauge from the circuit portion is formed on a main surface of the first silicon substrate. The LOCOS film is located outside an outermost peripheral portion of a thin-wall portion of the diaphragm portion.
Preferably, the circuit portion includes a plurality of circuit elements isolated from each other by a trench that penetrates the first silicon substrate in a thickness direction to reach the insulation film. A plurality of trenches may be provided in the first semiconductor substrate for isolating the circuit elements. In this case, the strain gauge should define a distance of 50 um or more from any one of the trenches on the main surface of the first silicon substrate.